Downshot burner

ABSTRACT

A burner outlet set for a downshot firing burner is described comprising a first outlet array having at least one primary outlet, and at least one vent air outlet disposed either side of the primary outlet in an array direction of the first outlet array; second and third outlet arrays each comprising an array of secondary air outlets, respectively disposed either side of the a first outlet array. A burner system with a plurality of such burner outlet sets, a burner arch configured for downshot firing and having one or more such burner sets, and a combustion furnace with one or more such arches are also described.

The present invention relates to burners adapted for downshot firing and further relates to combustion systems for furnaces for example for steam generators for example for utility power generation including such arrangements of burners. In particular, but not exclusively, the invention relates to burners/furnaces capable of utilising low volatile fuels, and especially low volatile carbonaceous solid fuels such as low volatile coals.

Low volatile coals such as anthracites and lean coals present particular requirements for effective combustion so as to overcome the inherent difficulties of achieving stable and efficient combustion which arise from the lack of volatile material in the coal to aid in the ignition, and the low reactivity of the remaining char. Downshot or arch firing is the established technology for low volatile coal combustion in utility power generation applications. Downwardly directed burners are mounted on one or more arches in an upper part of the combustion chamber to give a typically U-shaped or W-shaped downwardly directed flame with a long residence time for more effective combustion of low volatile fuels. Burners may be mounted on a single arch (U-shaped flame) or mounted on a pair of opposing arches (W-shaped flame). Generally single arch firing is used in lower thermal capacity (smaller) boiler and double arch firing (W-firing) is preferred for higher capacity larger boilers.

An example prior art downshot firing arrangement is shown in FIGS. 1 (furnace) and 2 (burner).

The firing arrangement of FIG. 1 is shown in a furnace of a thermal electric utility power plant. FIG. 1 illustrates the general furnace arrangement and principles which apply both to the prior art and to burner sets in accordance with the invention.

The fuel source is envisaged to be a low volatile carbonaceous fuel in pulverous form such as a low volatile coal in pulverous form, for example anthracite or lean coal: Coal entrained in a transport gas comprising transport air or other comburant gas is taken from the pulverizing plant (in the case of direct firing) or from a pulverised coal silo (indirect firing). In a typical system for downshot firing, the fuel/transport air mix is then separated into two streams in a separator which may be a cyclonic separator known routinely in the art as a relatively fuel rich “primary” stream containing most of the fuel (for example taken from the lower part of the cyclone) and a relatively fuel lean “vent” air stream (for example taken from the upper part of the cyclone). Following separation, the coal and air are introduced into the furnace via an arrangement of slots in the firing arch which additionally include “secondary” (windbox) air slots and possible further air slots. A possible arrangement is shown in FIG. 2.

FIGS. 1 and 2 are discussed in greater detail below.

The invention is directed to systems which have burner sets comprising “primary”, “vent”, and “secondary” nozzles as they would be understood by the skilled person in accordance with the above, and the description here should be read accordingly.

In particular references to primary air will in familiar manner generally be understood to refer to air or other comburant gas mixtures admitted to the system with the fuel upstream of the combustion zone to support combustion at the combustion zone in a burner or furnace and in the context of this invention therefore references to primary air will be understood to refer to the air or other comburant gas forming the relatively fuel rich stream after the separation process above described.

References to vent air will correspondingly be understood to refer to the air or other comburant gas forming the relatively fuel lean stream after the separation process above described.

References to secondary air will in familiar manner be understood to refer to air or other comburant gas mixtures admitted to the system separately from the fuel/primary mixture to support combustion at the combustion zone in a burner or furnace. For example secondary air is supplied via the windbox.

More generally, as is usual in the art, all references to “air” include air, simulated air, and other comburant gas mixtures capable of supporting combustion of the fuel. In a downshot firing arrangement such as that illustrated, fuel and combustion air is introduced into the furnace via burners situated on the firing arches. The inlet flows are directed downwards to the hopper. In the idealised situation a symmetrical “W” shaped flame pattern is developed, although in practice there is often significant asymmetry.

An increasing environmental pressure surrounds the control of production of nitrogen oxides (NOx) during combustion. The use of furnace air staging for NOx control is known for the firing of higher volatile coals, though the details of its implementation differ between designs. It has not been widely adopted for the firing of higher volatile coals in downshot fired systems. Air staging involves the removal of a proportion of the combustion air from the burner and this can have a detrimental effect on flame stability. Flame instability and asymmetry can be detrimental to NOx control, particularly in relation to furnace air staging methods.

There is a general desire to develop improved burner designs for the more effective downshot firing of low volatile carbonaceous fuels such as low volatile coals and in particular to develop improved downshot burner designs that are susceptible to effective NOx control. There is a particular desire to develop burner designs that produce more stable ignition and/or flame symmetry.

Thus in accordance with the invention in a first aspect, a burner outlet set for a downshot firing burner comprises:

a first outlet array comprising at least one primary outlet, and at least one vent air outlet disposed either side of the primary outlet in an array direction of the first outlet array;

second and third outlet arrays each comprising an array of secondary air outlets, respectively disposed either side of the first outlet array.

As the skilled person will appreciate, references to “primary”, “vent”, and “secondary” outlets will be understood in the context of the provision of comparable burner sets in the prior and the description here should be read accordingly.

References to primary outlets will be understood to refer to outlets for the air or other comburant gas forming the relatively fuel rich stream after the separation process above described. References to vent air outlets will correspondingly be understood to refer to outlets for the air or other comburant gas forming the relatively fuel lean stream after the separation process above described. References to secondary air outlets will in familiar manner be understood to refer to outlets for air or other comburant gas mixtures admitted to the system separately from the fuel/primary mixture to support combustion at the combustion zone in a burner or furnace. For example secondary air is supplied via a windbox.

A burner outlet set thus comprises a first outlet array disposed in an elongate first array direction and comprising at least one primary outlet with at least one vent air outlet disposed either side of the primary outlet, and paired second and third outlet arrays each comprising a plurality of secondary air outlets respectively disposed in elongate second and third array directions that are laterally spaced either side of the first outlet array. Typically the second and third outlet arrays are even laterally spaced either side of the first outlet array. Typically the second and third outlet arrays are arrayed in elongate array directions generally parallel to the first array direction. Typically each of the first, second and third outlet arrays extend in elongate array directions to the same longitudinal extent.

Each primary outlet is preferably a square or rectangular outlet disposed with a median line direction parallel to and for example coincident with the first array direction,

Each vent air outlet is preferably a square or rectangular outlet disposed with a median line direction parallel to and for example coincident with the first array direction.

Each secondary air outlet is preferably a square or rectangular outlet disposed with a median line direction parallel to a respective second or third array direction.

In a preferred embodiment, each primary outlet is associated with exactly one secondary outlet in the second array and exactly one secondary outlet in the third array. More preferably each primary outlet is aligned in a direction transverse to the array direction of the first outlet array with exactly one secondary outlet in the second array and exactly one secondary outlet in the third array. For example the outlets are so aligned in that their respective transverse midlines line on a single common transverse line. Thus, in such a case, each primary outlet is sandwiched in a transverse direction between a pair of secondary outlets. Optionally the secondary air may be biased towards one or other outlet. In such an arrangement, each primary outlet is thus completely surrounded by clean air or vent air.

In a preferred embodiment, each outlet in the first array is associated with exactly one secondary outlet in the second array and exactly one secondary outlet in the third array. More preferably each primary outlet is aligned in a direction transverse to the array direction of the first outlet array with exactly one secondary outlet in the second array and exactly one secondary outlet in the third array. For example the outlets are so aligned in that their respective transverse midlines line on a single common transverse line. Thus, in such a case, each outlet in the first array is sandwiched in a transverse direction between a pair of secondary outlets. Optionally the secondary air may be biased towards one or other outlet.

Although the invention is not limited by particular theory, it is generally suggested that efficient combustion of low volatile coats and in particular effective operation of furnace air staging as a method of NOx control requires gross flow patterns and flame paths in the lower furnace to be as symmetrical as possible. Whist flame asymmetry has a number of negative impacts on plant performance, it is of particular concern when considering in-furnace NOx reduction processes such as furnace air staging. It is postulated that the inconsistent flame stand off identified in prior art systems such as those shown in FIGS. 1 and 2 may be one of the factors contributing to the variable asymmetry that is found to be detrimental to performance.

From the above it follows that it is considered important to NOx control in downshot fired systems that the gross flow patterns and flame paths are broadly symmetrical, and that the flame paths and stand-offs are steady and repeatable. The key requirements of any improved burner design addressing these problems can be seen to include:

a long flame that penetrates into the lower furnace to utilize the furnace volume more fully and efficiently;

a flame whose ignition point is clearly defined in a robustly repeatable manner; a symmetrical flame path.

Burners comprising at least one burner outlet set embodying the principles of the invention are found to deliver these requirements in admirable manner.

In accordance with the invention a first outlet array comprises at least one primary/vent outlet module made up at least one primary outlet and at least one vent air outlet disposed either side of the primary outlet.

Such a primary/vent outlet module may comprise or consist of exactly a single primary outlet and a single vent air outlet disposed either side of the primary outlet, or a plurality of adjacent primary outlets with a single vent air outlet disposed either side of the primary outlet, or a single primary outlet with a plural set of vent air outlets disposed either side of the primary outlets or a plurality of adjacent primary outlets with a plural set of vent air outlets disposed either side of the primary outlets. In all cases the outlets making up such a vent outlet module are disposed in an elongate first array direction. Preferably, a vent outlet module comprises outlets configured and disposed such as to give mirror symmetry about a transverse midline of the module.

A first outlet array may comprise exactly one primary/vent outlet module as herein defined.

Alternatively a first outlet array may comprise more than one primary/vent outlet module arrayed adjacently. In such an embodiment a first outlet array thus comprises at least one first primary outlet with at least one vent air outlet disposed either side of the primary outlet, and at least one further primary outlet with at least one vent air outlet disposed either side of the primary outlet adjacent thereto in an array direction of the first outlet array. For example a first outlet array may comprise exactly one pair of adjacent primary/vent outlet modules as herein defined.

Each outlet is preferably a square or rectangular outlet disposed with a median line direction parallel to its elongate array direction. A rectangular outlet may be elongate in a direction parallel to its elongate array direction or in a direction transverse to its elongate direction.

The individual outlets making up the first outlet array may be identically or differently sized and/or shaped to each other. For example at least the primary outlets and the vent outlets making up the first outlet array may be differently sized and/or shaped from each other. In a preferred case a primary outlet may be larger than its corresponding vent outlets.

The outlets making up the first outlet array are preferably arrayed in aligned manner such that their respective median lines are aligned with each other along an elongate array direction.

The individual outlets making up the second outlet array may be identically or differently sized and/or shaped to each other. The individual outlets making up the third outlet array may be identically or differently sized and/or shaped to each other. In a preferred case, regardless of whether the individual outlets making up the respective outlet arrays are differently shaped, the second outlet array and the third outlet array are identically configured. In a preferred case each outlet in the second outlet array is aligned with exactly one identically sized and shaped outlet in the third outlet array.

The outlets making up the second outlet array are preferably arrayed in aligned manner such that their respective outermost edges are aligned with each other along an elongate array direction. The outlets making up the third outlet array are preferably arrayed in aligned manner such that their respective outermost edges are aligned with each other along an elongate array direction.

In a preferred embodiment, each outlet in the first array is aligned with exactly one outlet in the second array and exactly one outlet in the third array in that their median lines are aligned with each other in a transverse direction. Thus, in such a case, each outlet in the first array is sandwiched in a transverse direction between a pair of secondary outlets. Optionally the secondary air may be biased towards one or other outlet. Optionally the secondary outlets may be of identical or different size and/or shape to each other and of identical or different size and/or shape to the outlet in the first array. In the event that the outlets are differently shaped they are preferably still of the same longitudinal extent in an elongate array direction.

The definitions of the outlets as primary, vent air, and secondary air outlets will readily be understood by the skilled person with reference to prior art systems. The invention subsists not in a redefinition of these terms but in the arrangement on configuration of slots. The invention is not considered limited to particular definitions, but such definitions are advanced to guide the skilled person as to preferred embodiments of the invention.

Transport gas comprising a comburant gas comprising air or other comburant gas mixture entrains and transports fuel from a fuel supply zone. In the preferred case for example, where fuel is pulverised coal, transport gas entrains and transports coal from the coal pulverisers directly (direct firing) or from a pulverised coal silo (indirect firing).

Transport gas comprising a comburant gas and fuel is split into primary and “vent air” streams via a suitable separator which may include a cyclone system. A “vent air” stream may contain a majority of the air so separated for example 51 to 70% of the air and a small minority of the fuel for example below 20% of the coal. A “primary” stream may contain a minority of the air so separated for example 30 to 49% of the air and a substantial majority of the fuel for example at least 80%. A more complete combustion system in accordance with the principles of the invention accordingly comprises a separator adapted to effect such separation.

Separation of the fuel and transport air into a relatively fuel lean and a relatively fuel rich stream aims to further improve ignition and flame stability. Removal of air reduces the total mass of fuel stream, and therefore reduces the time for this primary stream to be heated to ignition temperature. Furthermore the higher concentration of fuel in the primary stream creates a mixture that is more conductive to combustion.

A more complete system in accordance with the principles of the invention accordingly further comprises one or more of:

a fuel supply source which is for example a supply of pulverous solid fuel, and for example includes a fuel pulveriser or a pulverised fuel store;

a transport gas supply source to supply comburant gas as a transport gas configured to entrain fuel from the fuel supply within the transport gas;

a transport conduit to transport the fuel entrained in the transport gas;

a separator configured to separate the mixture of fuel and transport gas into a relatively fuel rich primary stream and a relatively fuel depleted vent air stream;

a primary conduit to convey the primary stream to the primary outlets of a burner set in accordance with the principles of the invention;

a vent air conduit to convey vent air to the vent air outlets of a burner set in accordance with the principles of the invention;

a secondary air supply system including a secondary conduit to convey secondary air to the secondary air outlets of a burner set in accordance with the principles of the invention, and for example including a windbox.

A burner system may be provided comprising a plurality of burner outlet sets in accordance with the first aspect of the invention.

In a possible embodiment a primary outlet may be associated with a wedge shaped bluff-body stabilizer. Normally this will be slitted and withdrawn into the nozzle (i.e. will be a cavity bluff-body), but variants to the bluff-body arrangement may be considered. The wedge may have a notional nozzle blockage ratio of 40%. The slit preferably comprises no more than 10% of the width of the wedge. The orientation of the bluff-body will be such that the induced recirculation aims to draw in flue gas from the combusting vent air. The “V” of the wedge may be aligned so that it is perpendicular to the furnace front/rear walls. In an alternative embodiment the V” of the wedge may be aligned so that it is parallel with the front or rear wall, such that the induced recirculation draws in secondary air.

Optionally a primary outlet may contain four ignition teeth located at the corners of a square or rectangular slot.

The primary outlet and vent air outlets are typically be located in triples adjacent to each other; the vent air outlets will be on the edges of the group while the primary outlet will be in the middle.

In the preferred design each primary outlet and vent air outlet on a burner will be sandwiched by a pair of secondary air outlets (inner and outer). In this way the primary outlet will be completely surrounded by clean air or vent air. Optionally the secondary air will be biased towards the inner or outer air outlets. In this context the inner side is the side nearest the centre of the furnace and the outer side is the side next to the front or real wall.

The secondary air outlets in a burner will preferably be square or rectangular in shape; they will preferably be aligned with the primary and vent air outlets.

The secondary air flow will for example deliver an overall stoichiometry at the burner of ˜0.8, when including the sum of primary, vent, secondary, and thermal biasing air streams.

An oil light-up burner may be located adjacent to the primary and vent air nozzles; notionally there will typically be one light-up burner for each pair of primary nozzles. Flame scanning may follow conventional current practice.

In a more complete aspect of the invention, one or more burner sets are disposed on a burner arch configured for downshot firing of fuel in a combustion chamber of a combustion apparatus. A burner set is disposed on a burner arch configured for downshot firing with its respective outlet array directions aligned in a general elongate arch direction. Thus the first outlet array comprises in such an arch assembly a central array disposed in an elongate arch direction along a burner arch configured for downshot firing of a furnace. The respective second and third arrays comprises respective inner and outer arrays disposed in an elongate arch direction along a burner arch configured for downshot firing of a furnace respectively towards an inner and an outer edge of the arch.

The invention in accordance with this more complete further aspect of the invention by analogy comprises a burner arch configured for downshot firing of fuel in a combustion chamber of a combustion apparatus having at least one burner set in accordance with the first aspect of the invention disposed on the burner arch whereby combustion of the fuel is supported in the vicinity of the burner set during use.

Burners may be mounted on a single arch (for example to produce a U-shaped flame) or mounted on a pair of opposing arches (for example to produce a W-shaped flame).

According to the invention in a more complete aspect there is provided a combustion apparatus comprising:

a combustion chamber defined by one or more combustion chamber walls; at least one burner arch as above described configured for downshot firing of fuel in the combustion chamber;

at least one burner set as above described disposed on the burner arch configured for downshot firing whereby combustion of the fuel is supported in the vicinity of the burner set during use.

The general principles of such a combustion apparatus configured for downshot firing are established. The combustion apparatus of the invention is distinctly characterised by the provision of at least one burner set embodying the principles of the first aspect in the invention.

Preferably, the combustion apparatus is further characterised by the provision of air staging through the supply of overtire air to the combustion chamber. Accordingly a burner set is preferably provided in association with at least one outlet or series of outlets for overfire air (OFA). Each supply of overfire air may be directed into the combustion chamber at an angle to the horizontal of between 0 degrees and 45 degrees downwardly.

In a particular preferred case a burner set is provided with overfire air directed at first and second levels respectively below and above a flame zone within the combustion chamber. Thus a burner set is preferably provided in association with at least one outlet or series of outlets for overfire air respectively provided at each of first and second levels respectively below and above a flame zone within the combustion chamber.

For example a first overfire air outlet or series of outlets may be provided on a combustion chamber wall, and for example on the front and rear combustion chamber walls of a rectangular chamber, in the vicinity of and for example above and closely adjacent to a hopper knuckle of the combustion chamber.

For example a second overfire air outlet or series of outlets may be provided on a combustion chamber wall, and for example on the front and rear combustion chamber walls of a rectangular chamber, in the vicinity of and for example above and closely adjacent to the burner arch.

As has been noted above, the invention is distinctly characterised by the provision of at least one burner outlet set embodying the principles of the first aspect in the invention, and in particular with the specific outlet configuration and geometries set out hereinabove, by means of which improved and more symmetrical gross flow patterns can be achieved during downshot firing, and a flame with a stable and repeatable ignition point can more readily be created. It is intended that plural such burner sets may be provided into a suitably configured downshot firing system, and for example provided within a suitably configured burner arch in generally familiar manner and applying generally well established principles for downshot firing with which the person skilled in the art will be familiar.

For example, a burner arch may comprise multiple laterally spaced burner sets in accordance with the principles of the invention. A burner arch may comprise other outlets in familiar manner, for example to deliver wall air into the vicinity of the combustion zone, to deliver thermal biasing air into the centre of the combustion zone etc. A suitable example of a system for the introduction of thermal biasing air may be found in WO9801701 A1 incorporated herein by reference.

Plural burner sets on a burner arch may be identical and evenly spaced. Where plural burner sets are provided on a burner arch, the burner arch may be configured such as to exhibit at least mirror symmetry about a transverse midline. Conveniently, plural burner sets in accordance with the principles of the invention may be provided either side of a burner midline, with a central supply of thermal biasing air. The burner sets may be arranged on an arch in two discrete groups (“A” and “B” side) with a gap in the furnace centre to provide a measure of thermal biasing.

Thermal bias air slots may be installed between the burner groups to ensure separation, to enhance the cooling effect at the centre of the furnace, and to provide an additional measure of air staging. The individual thermal biasing air slots may notionally have the same air flow as the equivalent secondary air slots, and may be fed from the same windbox supply.

In familiar manner, symmetrically paired arches may be provided to produce a stable W-shaped flame in the combustion chamber during use.

Other features of these more complete aspects will be understood by analogy.

The combustion apparatus is in the preferred case a furnace for a steam generator in a thermal power plant which is fired by a plurality of burner sets in accordance with the first aspect of the invention.

Thus, in accordance with the most complete aspect of the invention, there is also provided a thermal power plant comprising at least one steam generator provided by thermal energy from a furnace fired by burner sets as above described, with suitable fuel supply means, and in fluid communication with suitable means to generate electrical power from the steam produced by the steam generator.

The invention will now be described by way of example only with reference to FIGS. 1 to 8 of the accompanying drawings in which:

FIG. 1 is a cut away side elevation of a prior art downshot fired furnace,/steam generator arrangement for a thermal power plant;

FIG. 2 is a plan view of a prior art burner outlet set of a burner of the downshot fired furnace/steam generator arrangement of FIG. 1;

FIG. 3 shows a possible arrangement for overfire air which may be applied generally not only to the prior art furnace illustrated in FIG. 1 but to a furnace modified to include burner sets in accordance with the invention such as might be exemplified in FIGS. 4-8;

FIGS. 4-8 show alternative example arrangements of burner sets on half burner arches embodying the principles of the first aspect of the invention.

An example prior art downshot firing arrangement is shown in FIGS. 1 (furnace) and 2 (burner).

In a downshot firing arrangement for a steam generator 1 such as that illustrated, fuel and combustion air is introduced into the furnace via burners 2 situated on the firing arches 3. The inlet flows are directed generally downwards to the hopper 4. Paired firing arches with equivalent burner arrangements are provided so that in the idealised situation a symmetrical “W” shaped flame pattern is developed. The flame geometry serves to give a flame with a long residence time for more effective combustion of low volatile fuels.

Because of the low volatile content of the coals typically fired in downshot plant, flame stabilization cannot rely upon the rapid release from volatiles combustion in the early part of the flame (as practised in conventional swirl stabilized circular burners for bituminous and low rank coals). Instead stabilization is achieved by the recycling of heat from the hot products of combustion through the following mechanisms:

radiation from the hot upward flowing combustion products into the cold downward flowing inlet coal and air (6);

radiation from hot refractory surfaces in the lower furnace into the cold inlet stream (7); refractory is typically installed on the firing arches and the front/rear walls below the arches;

entrainment of hot products of combustion from the upward flowing gases into the cold inlet stream (8).

The firing arrangement of FIG. 1 is shown in a furnace of a thermal electric utility power plant. The thermal energy is used to heat a process fluid and thereby drive a series of turbines in a generator in familiar manner, and the skilled person will readily infer the features of such a more complete thermal electric utility power plant system. The invention is applicable to the general furnace principles illustrated by FIG. 1, and is embodied in the detail of the burner set arrangements such as illustrated by example in FIGS. 4 to 8.

The fuel source is a low volatile carbonaceous fuel in pulverous form such as a low volatile coal in pulverous form, for example anthracite or lean coal.

Coal and transport air from the pulverizing plant is separated into two streams in a cyclone (10); a “vent air” stream 11 containing typically 60% of the air and 10% of the coal, and a “primary” stream 12 containing typically 40% of the air and 90% of the coal. Following separation, the coal and air are introduced into the furnace via an arrangement of slots in the firing arch and described in more detail with reference to FIG. 2, a series of slots each primary air/coal slot being located between a pair of longer secondary (windbox) air slots.

Separation of the coal and transport air into a fuel lean and a fuel rich stream aims to further improve ignition and flame stability. Removal of air from the coal reduces the total mass of fuel stream, and therefore reduces the time for this stream to be heated to ignition temperature. Furthermore the higher concentration of the coal creates a mixture that is more conducive to combustion. It is generally considered that for low volatile coals a particle concentration in the range 1.0˜1.5 kg/m³ gives the highest propagation velocities (i.e. best combustibility) (M Tarniguchi et al; “Comparison of flame propagation properties of petroleum coke ad coals of different rank”; Fuel 88 (2009), 1478-1484). In the described downshot system the concentration of coal in the primary stream is typically 1.3 kg/m³.

A tertiary air stream 14 is introduced on the front and rear walls just above the hopper knuckle.

Wall air, supplied from the secondary air windboxes, is introduced at the outside edge of the furnace arch via slots created by the removal of the membrane strip,

The various streams are introduced via a series of slots, as shown in FIG. 2. Following separation, the coal and air are introduced into the furnace via a series of slots, each primary air/coal slot being located between a pair of longer secondary (windbox) air slots. Vent air slots are additionally provided.

The slot dimensions are chosen to deliver the required inlet velocities; in particular the velocity differential between the primary and secondary streams facilitates the mixing between them.

In the particular arrangement of FIG. 2, part of a burner arch 20 is shown in simple schematic view carrying three burner sets 21 per half arch, and a central outlet 22 for thermal biasing air. This simple schematic is shown to the right of the figure. An inlet showing greater detail of the slotted burner set 21 is shown to the left of the figure.

In the illustrated burner set, four identical sets of primary/vent/secondary slots are shown, each comprising an elongate primary slot 25 and adjacent vent slot 26 with secondary slots 27 longer than and disposed either side of the primary slot 25. Wall air is supplied through the wall air slots 28. This is an example arrangement only. Similar arrangements with the primary and vent slots swapped can be considered so that either the vent slots or the primary slots may be next to the wall.

Although the general desire is to produce a symmetrical “W” shaped flame pattern in practice there is often significant asymmetry.

Typically the flames from one of the firing arches will extend down to the hopper region before turning up (the “long” flame path), while the flames on the opposite firing arch will turn up almost immediately and bypass the lower furnace (the “short” flame path). This behaviour will have an adverse effect on combustion efficiency as the short flame has a reduced residence time for burnout. It also raises concerns with in-furnace NOx reduction technology based on staging of the combustion air, where the reducing zone residence time is not well defined, and NOx reduction performance is likely to be compromised for the short flames. Finally the asymmetric flame pattern leads to significant imbalances in the pattern of heat absorption in the furnace and pendants, causing operational problems.

Some or all of these problems may be mitigated by arrangements of primary/vent/secondary outlets in accordance with the principles of the invention such as exemplified by the illustrated embodiments of FIGS. 4 to 8. In particular in the preferred case some or all of these problems may be mitigated by provision of a system in which such burner sets are further provided with overfire air (OFA), most preferably directed at first and second levels respectively below and above a flame zone within the furnace. The arrangement of level 1 OFA and level 2 OFA described with reference to FIG. 3 also aims to mitigate flame asymmetry.

FIG. 3 shows a possible arrangement for overfire air which is applicable in general principle to burner sets in accordance with the invention and may be viewed in the context of the general furnace design of FIG. 1. Overfire air nozzles are provided at two levels respectively below and above the flame.

Overfire air outlets (level 1) to produce the stream shown will be located across the front and rear wall width below the flame, for example just above the hopper knuckle. The air is angled downwards (in the embodiment notionally 30°˜45° below horizontal, although overfire air directed at any angle from horizontal to 45° below horizontal might be considered), and turning vanes may be installed in the approach duct to reinforce the downward direction. The air is typically introduced via a series of nozzles. An example notional 15% of the air may be supplied as level 1 OFA (equivalent to a stoichiometry of ˜0.2). As well as air staging, the intention of the level 1 OFA is to draw the flames down towards the hopper, and to turn the flames before they impinge in the hopper slope.

Overfire air outlets (level 2) to produce the stream shown will be located across the front and rear wall width just above the firing arch. The air is angled downwards (notionally 30°˜45′ below horizontal), and turning vanes may be installed in the approach duct to reinforce the downward direction. The air will be introduced via a series of nozzles. Notionally 15% of the air will be supplied as level 2 OFA (equivalent to a stoichiometry of ˜0.2). As well as air staging, the intention of level 2 OFA ports is to prevent the premature turning of the flames to reinforce the downward direction.

Air staging using conventional overfire air has not been widely adopted for the firing of lower volatile coals in downshot fired systems as it has been seen to have a detrimental effect on flame stability. However, in preferred embodiments of the invention, these overfire principles are effectively applicable to burner sets in accordance with the principles of the invention such as are exemplified in FIGS. 4 to 8.

All the embodiments exemplified in FIGS. 4 to 8 show a half burner arch 40 with three burner nozzle sets 41 each embodying the principles of the invention. Each example burner nozzle set has in common a central array of primary 45 and vent 46 air slots with inner 47 and outer 48 arrays of secondary air slots. The slots are square or rectangular, but need not be identically configured, although a burner set typically at least exhibits mirror symmetry about both a longitudinal and a transverse mid line as regards the disposition of these primary, vent air and secondary air slots. A central thermal biasing air outlet 42 is optionally provided.

Other outlets (not shown) for example to deliver wall air, may optionally be included.

The system comprises in familiar manner (not expressly shown) a supply of pulverised low volatile coal which is for example an on site pulverising plant and a supply of transport air. The transport air entrains the pulverised coal in familiar manner and transports the coal from the pulverising plant or other supply.

The supply of transport air and coal is then split into primary and vent air streams in accordance with suitable principles such as will for example be familiar from the prior art. For example the split between a primary stream and a vent air stream is effected in a cyclonic separator.

Coal and transport air from the pulverizing plant is separated into two streams in the cyclonic separator; a “vent air” stream containing typically 60% of the air and 10% of the coal, and a “primary” stream containing typically 40% of the air and 90% of the coal. Following separation, the coat and air are introduced into the furnace via an arrangement of slots in the firing arch via primary air (PA), vent air (VA) and secondary air (SA) slots in generally familiar manner, the invention being characterised by the particular arrangement/configuration of slots in accordance with the invention.

The invention is particularly characterised by the provision of burner sets in accordance with the principles of the first aspect of the invention, and such as might be exemplified by the embodiments of burner set illustrated in FIGS. 4-8. Other aspects of a more complete combustion system or furnace embodying the principles of the invention not specifically shown in the drawings or described herein will readily be inferred by the skilled person from general knowledge of prior art downshot fired systems, whether as generally exemplified by the system illustrated in figure or otherwise. The invention encompasses all downshot fired systems provided with burner shots embodying the principles of the first aspect of the invention such as are illustrated in the following figures.

In the embodiments shown a nozzle set provides a primary outlet via a single primary air (PA) slot 45 or a pair of primary slots 45 as the case may be. Each such primary slot or pair might for example be referred to as a burner outlet, Each such primary slot or pair (or burner outlet) has at least one vent air (VA) slot 46 at either side. Each primary or vent air slot 47, 48 in the central row is aligned with exactly one secondary air (SA) slot in a row transversely spaced on either side, not necessarily the same shape as the central slot but in all the example cases of the same width. Each burner outlet is thus completely surrounded by clean air or vent air.

The following short description applies to each proposed burner set configuration given by way of example.

The burner sets may first be categorised as grouped into one of two options as follows.

Option A. Two PA slots per burner.

Option B: Single PA slot per burner. In the suggested configurations of the embodiments such a PA slot can be made wider than the VA slots. SA slots are made with the same widths as the respective PA/VA slots to which they are aligned.

Each of the above options can then be sub-categorised based on the manner in which the burner sets are defined. One possibility is to assume each burner set has only one burner (ie only one primary slot or adjacent pair or other adjacent plural series of primary slots) while the other is to assume each burner set is composed of two burners (two separate primary slots, or pairs or series, in the context of the invention defined by and separated by vent air slots). Dual burner sets are the current established practice. Embodiments of the invention comprising two burners in a set (and hence in accordance with the principles of the invention two separate single primary slots, or adjacent pairs other adjacent plural series of primary slots, separated by vent air slots) are likely to be preferred.

Another important consideration for arranging burners within the firing arch is to ensure they are better distributed as it is beneficial for the NOx performance (plant operational experience indicates that uniform distribution of burners within the firing arch, hence uniform heat release, gives lower NOx emissions than grouped and separated burner sets).

FIGS. 4 to 8 illustrate examples only of burner sets which embody these principles.

In FIG. 4, a burner set comprising dual burners is shown. Each burner comprises a pair of primary slots 45 disposed (with reference to a direction longitudinal to the arch) between a pair of vent air slots 46. Both the primary and the vent air slots are rectangular and elongate in a direction transverse to the arch, with the primary slots elongate to a greater extent. The primary and vent air slots form a central array defining an array direction coincident with the respective median lines of the rectangles.

Each primary and vent air slot is paired on either side in a transverse direction by respective inner 47 and outer 48 secondary slots. The secondary slots make up respectively the second and third rows of slots so that each slot in the central row is disposed between secondary slots on either side in a transverse direction. The secondary slots are also rectangular, identical to each other, and identical in longitudinal extent along the arch to the longitudinal extent of the respective slots within the central row with which they are paired.

The arrangement produces a situation in which each burner outlet, in this case made up of a pair of adjacent primary slots, is entirely surrounded by vent air or secondary slots. Two such burners are provided in the illustrated embodiment.

Having regard to operational requirements and plant geometrical constraints, such an arrangement might be too crowded for many operational scenarios, and accordingly alternative embodiments such as those exemplified in FIGS. 5 to 8 might be explored instead.

In FIG. 5, a burner set comprising a single burner is shown. The burner comprises a single primary slots 45 disposed between a pair of vent air slots 46. Both the primary and the vent air slots are rectangular and elongate in a transverse direction, with the primary slots elongate to a greater extent. The primary and vent air slots form a central array defining an array direction coincident with the respective median lines of the rectangles.

Each primary and vent air slot is paired on either side in a transverse direction by respective inner 47 and outer 48 secondary slots. The secondary slots make up respectively the second and third rows of slots so that each slot in the central row is disposed between secondary slots on either side in a transverse direction. The secondary slots are also rectangular, and identical in longitudinal extent along the arch to the longitudinal extent of the respective slots within the central row with which they are paired, but in this case are of different lengths as shown. The arrangement still maintains the principle however that the primary slot is entirely surrounded by vent air or secondary slots.

In FIG. 6, a burner set comprising dual burners is shown. Each burner comprises a single primary slot 45 disposed between a pair of vent air slots 46. Each primary and vent air slot is again paired on either side in a transverse direction by respective inner 47 and outer 48 secondary slots.

In FIG. 7, a burner set comprising a single burner is shown. The burner comprises a single primary slot 45 disposed between a pair of vent air slots 46. Each primary and vent air slot is again paired on either side in a transverse direction by respective inner 47 and outer 48 secondary slots. It differs from the FIG. 5 embodiment in that the outlet slots have a different aspect ratio.

In FIG. 8, a burner set comprising dual burners is shown. Each burner comprises a single primary slot 45 disposed between a pair of vent air slots 46. Each primary and vent air slot is again paired on either side in a transverse direction by respective inner 47 and outer 48 secondary slots. It differs from the FIG. 6 embodiment in that the outlet slots have a different aspect ratio.

All these alternative arrangements maintain the principle that the primary slot is entirely surrounded by vent air or secondary slots. Although the invention is not limited by particular theory, it is generally suggested that such an arrangement can assist in supporting a flame whose ignition point is clearly defined in a robustly repeatable manner and encouraging a symmetrical flame path. 

1. A burner outlet set for a downshot firing burner comprising: a first outlet array comprising at least one primary outlet, and at least one vent air outlet disposed either side of the primary outlet in an array direction of the first outlet array; second and third outlet arrays each comprising an array of secondary air outlets, respectively disposed laterally either side of the a first outlet array,
 2. A burner outlet set thus comprises a first outlet array disposed in an elongate first array direction and comprising at least one primary outlet with at least one vent air outlet disposed either side of the primary outlet, and paired second and third outlet arrays each comprising a plurality of secondary air outlets respectively disposed in elongate second and third array directions that are laterally spaced either side of the first outlet array. Typically the second and third outlet arrays are even laterally spaced either side of the first outlet array. Typically the second and third outlet arrays and arrayed in elongate array directions generally parallel to the first array direction. Typically each of the first, second and third outlet arrays extend in elongate array directions to the same longitudinal extent.
 3. A burner outlet set in accordance with claim 1 wherein each primary outlet is a square or rectangular outlet disposed with a median line direction parallel to and for example coincident with the array direction of the first outlet array.
 4. A burner outlet set in accordance with claim 1 or claim 2 wherein each vent air outlet is a square or rectangular outlet disposed with a median line direction parallel to and for example coincident with the array direction of the first outlet array.
 5. A burner outlet set in accordance with any preceding claim wherein each secondary air outlet is a square or rectangular outlet disposed with a median line direction parallel to and laterally spaced from the array direction of the first outlet array.
 6. A burner outlet set in accordance with any preceding claim wherein each primary outlet is associated with exactly one secondary outlet in the second array and exactly one secondary outlet in the third array.
 7. A burner outlet set in accordance with claim 6 wherein each vent air outlet is associated with exactly one secondary outlet in the second array and exactly one secondary outlet in the third array.
 8. A burner outlet set in accordance with claim 7 wherein each outlet in the first array is transversely aligned with exactly one secondary outlet in the second array and exactly one secondary outlet in the third array.
 9. A burner outlet set in accordance with any preceding claim wherein a first outlet array comprises at least one primary/vent outlet module consisting exactly of a single primary outlet and a single vent air outlet disposed either side of the primary outlet.
 10. A burner outlet set in accordance with any preceding claim wherein a first outlet array comprises at least one primary/vent outlet module consisting exactly of a pair of primary outlets and a single vent air outlet disposed either side of the primary outlet.
 11. A burner outlet set in accordance with claim 9 or 10 wherein a first outlet array comprises exactly one said primary/vent outlet module.
 12. A burlier outlet set in accordance with claim 9 or 10 wherein a first outlet array comprises exactly two of the said primary/vent outlet modules disposed adjacently.
 13. A burner outlet set in accordance with any preceding claim wherein each primary outlet is provided with a wedge shaped bluff-body stabilizer.
 14. A burner outlet set in accordance with claim 13 wherein the bluff body stabilizer is slitted and located in a position withdrawn into its outlet.
 15. A burner outlet set in accordance with any preceding claim wherein each primary outlet is a square or rectangular slot provided with four ignition teeth located at the corners of the square or rectangular slot.
 16. A burner system comprising a plurality of burner outlet sets in accordance with any preceding claim.
 17. A fuel combustion system comprising: a pulverous solid fuel supply source; a transport gas supply source to supply comburant gas as a transport gas; a mixing module configured to entrain fuel from the fuel supply within the transport gas; a transport conduit to transport the fuel entrained in the transport gas; a separator configured to separate the mixture of fuel and transport gas into a relatively fuel rich primary stream and a relatively fuel depleted vent air stream; a primary conduit to convey the primary stream to primary outlets of a burner outlet set in accordance with any preceding claim; a vent air conduit to convey vent air to vent air outlets of a burner outlet set in accordance with any preceding claim; a secondary air supply system including a secondary conduit to convey secondary air to the secondary air outlets of a burner set of a burner outlet set in accordance with any preceding claim.
 18. A burner arch configured for downshot firing of fuel in a combustion chamber of a combustion apparatus having at least one burner outlet set in accordance with one of claims 1 to 15 disposed on the burner arch whereby combustion of the fuel is supported in the vicinity of the burner set during use,
 19. A combustion apparatus comprising: a combustion chamber; at least one burner arch configured for downshot firing of fuel in the combustion chamber; at least one burner outlet set in accordance with one of claims 1 to 15 disposed on the burner arch whereby combustion of the fuel is supported in the vicinity of the burner set during use.
 20. A combustion apparatus in accordance with claim 19 further configured for the supply of overfire air to the combustion chamber in that each burner set is provided in association with at least one outlet or series of outlets for overfire air.
 21. A combustion apparatus in accordance with claim 20 wherein each burner set is provided in association with at least one outlet or series of outlets for overfire air respectively provided at each of first and second levels respectively below and above a flame zone within the combustion chamber.
 22. A combustion apparatus in accordance with claim 21 wherein a first overfire air outlet or series of outlets is provided above and closely adjacent to a hopper knuckle of the combustion chamber, and wherein a second overfire air outlet or series of outlets is above and closely adjacent to the burner arch.
 23. A combustion apparatus in accordance with claims 19 to 22 further comprising: a pulverous solid fuel supply source; a transport gas supply source to supply comburant gas as a transport gas configured to entrain fuel from the fuel supply within the transport gas; a transport conduit to transport the fuel entrained in the transport gas; a separator configured to separate the mixture of fuel and transport gas into a relatively fuel rich primary stream and a relatively fuel depleted vent air stream; a primary conduit to convey the primary stream to the primary outlets; a vent air conduit to convey vent air to the vent air outlets; a secondary air supply system including a secondary conduit to convey secondary air to the secondary air outlets. 